Processes for producing alkyl tert-alkyl ethers by reacting a primary alcohol with a tertiary olefin having from 4 to 7 carbon atoms over a suitable catalyst are well known in the art. Two representative ethers which have considerable commercial success as motor fuel octane enhancers are methyl tert-butyl ether (MTBE) and tert-amyl methyl ether (TAME) which are made by reacting isobutylene and isopentenes, respectively, with methanol.
Etherification reactions are exothermic and equilibrium-limited and are generally carried out in the liquid phase in catalytic reactors having one or two fixed beds wherein heat is removed by circulating liquid through external heat exchangers. Etherification catalysts are generally strongly acidic ion exchange resins.
Tertiary olefin conversions obtained in conventional etherification reactions employing a single reactor wherein excess methanol is utilized, are generally limited to within the range of 90 to 96%. To achieve higher conversion, a two-stage reactor system using between 2 and about 20% excess methanol is generally practiced. The use of excess alcohol also suppresses polymerization of olefins to dimers and trimers. Unfortunately, methanol forms azeotropic mixtures with ethers and C.sub.4 to C.sub.7 hydrocarbons wherein separation by ordinary distillation is very difficult and, as a result, both energy- and capital-intensive.
Several techniques have been disclosed in the prior art for removing methanol from azeotropic etherification products. For example, U.S. Pat. No. 3,726,942 discloses a MTBE process in which the MTBE effluent stream is first sent to a distillation column to separate MTBE (bottom product) from the C.sub.4 hydrocarbon (overhead product). The crude MTBE product is water washed to remove methanol. The crude C.sub.4 hydrocarbon product is also water washed to remove methanol; alternatively, mol sieve is used to remove methanol. The methanol-water mixture is then separated by distillation and the methanol fraction is recycled to the MTBE reactor.
Patent Application EP-205562 discloses a process for preparing methyl-tert-alkyl ether which comprises: (a) contacting and reacting in the liquid phase a reaction mixture formed by combining a stream consisting essentially of C.sub.4 -C.sub.5 hydrocarbons and containing at least some proportion of isoalkylene and a stoichiometric excess of methanol, with respect to the isoalkylene, to form a reaction product comprising methyl-tert-alkyl ether, unreacted methanol and unreacted C.sub.4 -C.sub.5 hydrocarbons; (b) isolating the methyl-tert-alkyl ether from the reaction product and (c) recovering the unreacted methanol from the residual portion of the reaction product; the improvement which comprises selectively adsorbing the methanol constituent of said residual reaction product in a bed of crystalline molecular sieve adsorbent and recovering the same by desorption using the C.sub.4 -C.sub.5 hydrocarbon, used to prepare the initial reaction mixture, as a purge-desorption stream.
French Patent 2,448,521-A discloses a process for producing ethers which comprises (a) reacting an alcohol with a tertiary olefin in the presence of an acid catalyst to produce an effluent containing ether, unreacted alcohol and unreacted hydrocarbons; (b) contacting the effluent with a molecular sieve capable of adsorbing the alcohol but not the ether; (c) distilling the non-adsorbed product to separate the ether from the hydrocarbons; (d) periodically interrupting step (b) and contacting the alcohol-laden molecular sieve with a stripping gas at a temperature sufficient to desorb the alcohol; and (e) adjusting the temperature and pressure of the effluent gas from step (d) to condense the alcohol. The process is stated to be especially useful for producing methyl tert-butyl ether.
U.S. Pat. No. 4,409,421 discloses a process for preparing a pure tertiary olefin in which an alkanol and an alkyl-tert-alkyl ether are separated by distillation followed by adsorption using synthetic ion exchange resins. U.S. Pat. No. 4,447,653 discloses a process for regenerating adsorbents used in an integrated process for producing ethers such as methyl tert-butyl ether. The regeneration procedure includes contacting the adsorbent with a portion of the treated hydrocarbon stream. The resultant contaminated hydrocarbon stream is passed into a stripping column used to remove the lights from the effluent of a dehydrogenation zone in which the isoolefin fed to the etherification zone is produced. The hydrocarbonaceous compounds collected on the adsorbent are thus recycled rather than being destroyed or lost in low purity effluent streams. The contaminated hydrocarbon stream may also be passed directly into the etherification zone.
U.S. Pat. No. 4,605,787 discloses a process for preparing MTBE which comprises reacting in vapor phase at a temperature between 170.degree. and 220.degree. F., methanol with isobutylene in the presence of ZSM-5 or ZSM-11 acidic zeolite catalyst characterized by having a constraint index of from about 1 to 12 and a silica/alumina ratio of at least 5. Removal of any excess methanol is accomplished by passing the reaction product through a bed of small pore zeolite.
U.S. Pat. No. 4,774,365 discloses an improved process for separating alcohols from ethers and/or hydrocarbon mixture in an etherification process. The excess alcohol reactant, which forms azeotropic mixture with the product ether and unreacted C.sub.4 -C.sub.7 hydrocarbons, is removed by passing the liquid azeotropic mixture over a pervaporation membrane which effectively breaks the azeotrope and permeates the alcohol with high flux and high selectivity.
A need exists in the art for an energy efficient, less capital intensive process for making alkyl tert-alkyl ethers, and more particularly, to an energy efficient cyclic process for removing unreacted alcohol from the azeotropic etherification reaction product mixture.